1. Field of the Invention
The present invention relates to a silver halide emulsion and a method of preparing the same. More particularly, the present invention relates to a highly sensitive silver halide emulsion whose fog increase during storage is slight, and a method of preparing such a silver halide emulsion.
2. Description of the Related Art
Strenuous efforts have been exerted to now toward the sensitivity enhancement for silver halide photosensitive materials. In silver halide photographic emulsions, the sensitizing dye adsorbed on the surface of silver halide grains absorbs light having been incident on the photosensitive material and transfers light energy thereof to silver halide grains, thereby exhibiting a photosensitivity. Accordingly, in the spectral sensitization of silver halides, it is contemplated that the enhancement of spectral sensitivity can be attained by realizing adsorption of a large amount of sensitizing dye on silver halide grains so as to increase the light absorption and thus increasing the light energy transferred to silver halides.
Consequently, in recent years, it is common practice to use emulsions wherein the configuration of silver halide grains is tabular so as to have a large surface area per volume. This idea of using tabular grains is common knowledge in the art to which the invention pertains (see, for example, U.S. Pat. No. 4,956,269).
For increasing the surface area of grains, it is needed to reduce the thickness of tabular grains. The reduction of the thickness tends to be disadvantageous from the viewpoint of stability of grain configuration, light scattering and monodispersity of grain size. Thus, the reduction of the thickness would tend to bring about inefficiency inhibiting the sensitivity enhancement. When the grain thickness falls in the region of 0.2 μm or less, there would occur such a situation that the sensitivity enhancement corresponding to an increase of surface area despite the reduction of grain thickness cannot be easily accomplished. The cause thereof would be relevant to the above-mentioned trend.
In the meantime, there has been proposed a concept of increasing the amount of sensitizing dye super-imposed per unit surface area by effecting adsorption in multiple layers (hereinafter referred to as “multilayer adsorption”) of a sensitizing dye which has conventionally been adsorbed on silver halide grains in a single layer (see, for example, Jpn. Pat. Appln. KOKAI Publication No. (hereinafter referred to as “JP-A-”) 10-239789). In the proposed method, it is not necessary to reduce the grain thickness to an extreme extent, so that it is easy to avoid the aforementioned inefficiency attributed to the deteriorations of stability of grain configuration, light scattering and monodispersity of grain size.
However, it is not an easy task to achieve stable multilayer adsorption of sensitizing dyes and efficient transfer of absorbed light energy to silver halide grains. Thus, a large number of investigations have been made on this matter. In particular, in recent years, the sensitivity enhancement by a multilayer adsorption of a combination of specified cationic dye and anionic dye has been attempted (see, for example, JP-A-2000-89405, and European Patent Publication (hereinafter also referred to as “EP”) 0985965A, EP's 1085373A and 1199595A).
In the methods thereof, however, the interaction between the sensitizing dye of the first layer having directly been adsorbed on silver halide grains and the dye layers superimposed on the first layer is so weak that expected sensitivity cannot be realized.
As means for solving this problem, there is disclosed adding of sensitizing dyes in the form of a dispersion containing a surfactant or the form of an oil drop dispersion (see, for example, JP-A-2002-49113). However, even if this means is applied, the level of improvement in the above problem is not satisfactory.